1. Field of the Invention
The present invention relates to an image forming apparatus comprising a photosensitive unit.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, an image of a document set on a document base is optically read, an electrostatic latent image corresponding to the read image is formed on the surface of a photosensitive drum, and the electrostatic latent image is developed (visualized) by a developer (toner and carrier) and printed on a paper sheet. The electrostatic latent image on the photosensitive drum is formed by a laser beam emitted from an exposure unit.
A developing roller, developer, paper sheet, and cleaning unit blade are brought into contact with the surface of the photosensitive drum. As a result of this contact, the characteristics of the surface of the photosensitive drum change with time, and it eventually becomes impossible to form an appropriate image. That is, the photosensitive drum has a limited life.
Therefore, the lifetime of the photosensitive drum is judged based on the number of printed paper sheets, and an expired photosensitive drum needs to be replaced with a new photosensitive drum.
However, there are various forms in image formation. Therefore, it is difficult to exactly judge the lifetime only by the number of printed paper sheets.
On the other hand, as a method for judging the lifetime of the photosensitive drum, there has been an example described in Jpn. Pat. Appln. KOKAI Publication No. 2002-82578. In this example, the photosensitive drum is charged and exposed, the charging potential and exposure potential of the photosensitive drum at this time are detected, and the charging potential and exposure potential are corrected in such a manner that the difference between both detected potentials is constant. The time at which to change the photosensitive drum is predicted based on these corrected amounts. It is to be noted that in the above-described publication, a method in which the time at which to change the photosensitive drum is predicted from the operation time of the drum, and a method in which the time at which to change the photosensitive drum is predicted from the operation time of the developing roller brought into contact with the drum have also been described.
However, in the method in which the charging and exposure potentials are corrected, respectively, to thereby predict the change time, a long time is required in the correction. Therefore, there is a problem that a long time is required for the judgment of the lifetime of the photosensitive drum. Since there are various forms in the image formation, it is difficult to exactly judge the lifetime only by the operation time of the photosensitive drum or the developing roller.